Rock drilling devices of the type described here, intended for drilling in rock, are fluid driven, most often hydraulically. An example of a rock drilling device according to such prior art technology is illustrated schematically in FIG. 1. The drilling device 1 can be connected to a fluid container, such as a tank 2 of hydraulic liquid. A pump 3 is used to create a source of hydraulic liquid under high pressure. A slide valve 4 controls, in interaction with control devices in a piston housing 7 and on the hammer piston 6, the hydraulic liquid such that at least one driving surface 5 of the hammer piston that runs in a piston housing in the drilling device is subject alternately to high pressure and to low pressure.
The hammer piston 6 is arranged such that it impacts at its forward end, the piston tip 8, onto the shank 10 of a drill adapter 9. A drill rod can be connected to the drill adapter 9 for the intended drilling into a surface to be drilled, such as into rock. Several drill rods can be connected together to form a drill string of such a length that the desired depth of drilling can be achieved. A control conduit 11a is present in the piston housing 7, which control conduit is arranged in connection with the source 3 of hydraulic liquid. This control conduit 11a interacts with a control chamber 12 formed between the hammer piston 6 and the piston housing 7, whereby the slide 4 can be controlled depending on the position of the hammer piston 6 in the axial direction relative to the piston housing 7. A conduit 11b exerts constant pressure onto a control edge of the hammer piston 6 for driving the piston backwards.
In order to maintain the drill rod in constant contact with the surface to be drilled and in order to maintain the parts of the drill string in constant contact with each other, a recoil damper, with a recoil piston 13 included, is arranged. This recoil piston 13 is normally arranged concentrically around the front part of the hammer piston 6. The recoil piston 13 is held pressed against the shank 10 of the drill adapter 9 by means of hydraulic liquid from a pressure conduit 14 that is arranged in contact with a high-pressure source through a constant-flow valve, such that the hammer piston 6 can impact against a non-elastic surface when it impacts onto the shank of the drill adapter.
The complete drilling device is pressed during drilling against the object to be drilled with a feed force. The feed force can be applied, for example, hydraulically in a drill rig, which is an equipment for setting the position and angle of one or several drilling devices while drilling. The drilling device is then often mounted on a carriage that can be displaced along a feed beam in the drill rig. If the feed force becomes greater than the recoil pressure, i.e. the product of the pressure in the liquid that drives the damper piston forward in the direction of drilling and the cross-sectional area of the recoil piston, or—to be more accurate—the driving surface of the recoil piston on which the liquid acts, then the recoil piston will be pressed backwards. In order to counteract this and to achieve as far as possible constant conditions when the hammer piston impacts onto the drilling steel or the shank adapter, a drainage conduit or balance conduit 16 has been arranged, which functions as described below.
Instead of the recoil piston 13 making direct contact with the shank 10 of the drill adapter 9, a bushing 15 can be placed in the damper between the recoil piston 13 and the shank 10 of the drill adapter 9, as is shown in, for example, the document U.S. Pat. No. 5,479,996. The recoil piston 13 has an additional function, which is that of absorbing recoil forces from the surface to be drilled when the drill steel is pressed against this surface with the impact force that is transmitted from the hammer piston 6. The recoil piston 13 absorbs the pressure that is transmitted back from the surface to be drilled hydraulically, and thus it oscillates in the axial direction controlled by the pressures to which is subject from hydraulic liquid and from the recoil forces from the drill steel. The recoil piston 13 is for this reason provided with a drive chamber 14b formed between the recoil piston and the piston housing. This drive chamber is limited by at least one forward driving surface 13b in the recoil piston. The drive chamber 14b is drained through a balance conduit 16 in the piston housing 7 when the recoil piston reaches a position that is sufficiently far forward. If the recoil piston 13 is driven backwards, such that the driving surface 13b becomes located behind the balance conduit 16, then the pressure in the drive chamber 14b will rise, whereby the pressure on the driving surface 13b entails the recoil piston 13 being driven forwards. If, on the other hand, the recoil piston 13 is driven forwards such that the driving surface 13b frees the opening of the balance conduit 16 with respect to the drive chamber 14b, then the drive chamber will be drained through the balance conduit 16, whereby the pressure in the drive chamber will 14b fall, which in turn entails the piston being pressed backwards. The recoil piston will in this way take up a position that balances around the point at which the driving surface 13b of the recoil piston opens the drive chamber 14b for the balance conduit 16.
One problem with the technology described above is that the function of the impact mechanism tends to be unstable in some devices, particularly when dimensioning for high rates of impact, and particularly after a certain period of operation.